Self-supporting non-wrapped filters prepared from filamentary materials such as cellulose acetate have been known for many years as indicated, for example, by U.S. Pat. Nos. 2,688,380, 3,079,930, 3,111,702, 3,190,294, 3,313,306 and 3,361,137. Although such filters are capable of providing economic benefits due to the elimination of the paper wrap that is normally used in the manufacture of fibrous filter rods, non-wrapped fibrous filters have not enjoyed appreciable commercial acceptance due largely to adhesion problems associated with attachment of the filter rod to a tobacco rod by conventional techniques currently used in the art. These conventional techniques involve the use of tipping paper coated with aqueous adhesives. When such techniques are used in the manufacture of filter cigarettes incorporating non-wrapped fibrous filters, a satisfactory product may be obtained at low production rates (i.e., rates of about 1,000 to 1,500 filter cigarettes per minute), but the product becomes totally unsatisfactory as production rates increase to the 4,000 cigarettes per minute and higher levels associated with present day cigarette manufacturing machines. Thus, the degree of success which has been achieved by non-wrapped fibrous filters has been confined, for the most part, to the production of multiple filters wherein a non-wrapped fibrous filter is joined to one or more other components by means of a combining wrap to form a composite wrapped filter. The combining wrap in composite filters eliminates the adhesion problems associated with non-wrapped fibrous filters but it also nullifies one of the principal economic benefits to be derived from non-wrapped filters, namely, the savings associated with elimination of the paper wrap.
Apart from the savings realized by eliminating the paper wrap from fibrous filter plugs, it is apparent that non-wrapped fibrous filters can also provide advantages in the manufacture of ventilated filter cigarettes. These potential advantages of the non-wrapped fibrous filters, however, have not led to their significant use in the manufacture of ventilated filter cigarettes due, again, to the adhesion problems associated with attachment of the filter rod to a tobacco rod with tipping paper. Indeed, the adhesion problems become even more acute if an adhesive-free zone is used in the area surrounding the ventilation openings in the tipping paper that is employed to attach the filter rod to the tobacco rod.
The adhesion problems relating to non-wrapped fibrous filters have been a factor in the development of various alternative techniques for manufacturing ventilated fibrous filter cigarettes. The most widespread technique used to date employs a perforated tipping paper in combination with a filter plug wrap that is highly porous. The inherent porosity of the paper used for wrapping the filter plug is sufficiently great so that it will not be a limiting factor in the degree of air dilution achieved by the filter plug wrap/perforated tipping paper combination. A similar arrangement involves the use of a relatively non-porous filter plug wrap that is provided with an excessive number of perforations which underlie the perforations in the tipping paper. In order to avoid obstruction of the perforations in the tipping paper with adhesive, an adhesive-free zone is ordinarily provided in the area surrounding the perforations. This leads, however, to an unsealed portion of the seam which joins the overlapping ends of the tipping paper and provides an unwanted path for additional air to be admitted into the fibrous core of the filter. Elimination of this unwanted path necessitates the application of a narrow strip of adhesive along the seam edge of the tipping paper by means of a carefully timed device which must be maintained precisely in registration with each cut segment of tipping paper. Consequently, one of the major disadvantages of this method is the great difficulty in obtaining consistent application of adhesive to the tipping paper at high production rates in order to achieve a uniform degree of ventilation in the resulting product.
The fact that adhesives used for attaching tipping paper to filter plugs can obstruct the ingress of dilution air forms the basis of another technique that has been proposed for manufacturing ventilated fibrous filter cigarettes. This technique employs inherently porous tipping paper in combination with porous filter plug wrap and the degree of ventilation is controlled by predetermined adhesive-free zones between the tipping paper and filter plug wrap. This method of air dilution is also not completely successful due to difficulty in achieving uniform porosities in both the tipping paper and filter plug wrap and to problems associated with the patterned application of aqueous adhesives to the tipping paper. Also, the porous tipping paper allows adhesive to bleed through the paper and leads to build-up of adhesive on machine parts contacted by the assembled filter cigarettes. This requires deposition of a barrier film to the tipping paper prior to application of the adhesive which adds to manufacturing costs and introduces an additional variable that must be carefully controlled in order to achieve a uniform degree of air dilution in the finished product.
Yet another approach to manufacturing ventilated fibrous filter cigarettes involves the introduction of perforations into the tipping paper and filter plug wrap after the filter rod has been attached to the tobacco rod. The perforations may be mechanically introduced by rolling the assembled filter cigarette along a predetermined path that is provided with needle-like elements which perforate the tipping paper and filter plug wrap circumferentially with the desired number of perforations appropriately located. Typical apparatus for mechanically perforating assembled cigarettes is disclosed in U.S. Pat. No. 4,140,137. Alternatively, the perforations may be formed by a suitable laser beam technique such as that described in U.S. Pat. No. 4,121,595. While these perforating methods largely avoid the problem of adhesive blockage of the perforations, the methods are not without their disadvantages. The use of needle-like elements, for example, leads to unacceptable variation in the size of the perforations due to rapid erosion of the elements under the conditions of use. Frequent replacement of the needle-like elements is, therefore, required and this results in periodic removal from service of the production apparatus while repairs are made. The laser perforating system, on the other hand, is capable of maintaining a high degree of uniformity in the formed perforations; however, present experience indicates that frequent servicing of the system is required apparently due to vaporized filter materials and degradation products which tend to condense on the focusing lenses of the laser system and other parts of the apparatus.
In spite of the considerable promise associated with non-wrapped fibrous filters and particularly to the use of such filters in ventilated filter cigarette products, it is clear that adhesion problems relating to their use have prevented significant commercial use of non-wrapped fibrous filters. These problems have, in fact, prompted those skilled in the art to seek alternative forms of fibrous filter cigarettes which are often more costly to produce or which have deficiencies in quality or performance as compared with non-wrapped fibrous filter cigarettes.